The treatment processes for municipal and industrial wastewater have evolved due to identification of harmful environmental conditions created by nitrogen rich wastewater treatment plant effluents in rivers and estuarian environments and subsequent and necessary regulatory changes required to protect the environment. Biochemical Oxygen Demand (BOD), ammonia-nitrogen, nitrite-nitrogen, nitrate-nitrogen and organic-nitrogen are now commonly restricted components of the effluents of wastewater treatment systems. Although biological wastewater treatment systems are routinely engineered to remove the BOD, ammonia-nitrogen and much of the organic-nitrogen from wastewater streams, in doing so, these wastewater treatment plants, known as nitrification systems, create a nitrate rich effluent that has been recognized as being harmful to some aquatic environments.
There are biological systems designed to remove the nitrate-nitrogen from nitrification system effluents and the process of biological nitrate-nitrogen removal is called de-nitrification. There are a variety of process configurations for de-nitrification and many of these may be incorporated directly within the conventional biological nitrification system. For example, in the most common approach to de-nitrification, the engineer establishes a de-nitrification reactor or reactors, also called an anoxic reactor(s) as the first reactor(s) in a series of separate reactors in which the latter reactors are operated aerobically with oxygen or air present with the intent to biologically oxidize the ammonia-nitrogen and organic-nitrogen to form nitrate-nitrogen. The first reactor is termed anoxic because no elemental oxygen or air containing oxygen is introduced for aeration or mixing in that reactor even though the anoxic reactor is also rich in organic matter because it receives the influent wastewater.
In the anoxic/aerobic reactor scheme described above the nitrate-nitrogen would exit the last tank to contaminate the environment and thus in processes with the anoxic/aerobic sequence, a large stream of the nitrified wastewater is pumped from the nitrification reactor back to the anoxic reactor where-in the nitrate-nitrogen is used as a source of nitrate-oxygen for facultative heterotrophic bacteria which use it processing the BOD of the incoming wastewater in the anoxic tank.
In the anoxic de-nitrification/aerobic nitrification process described above, a means of mixing the influent wastewater with the large recycled stream of nitrate rich water is critical to the effective contact of the mixture with the bacteria. Wherein the overall system is operating as an activated sludge system with suspended bacteria a mechanical mixing device is sufficient, albeit it requires more energy and capital investment. In some instances the series of reactors, both anoxic and aerobic, may use fixed-film biological populations that grow on surfaces fixed within the reactor vessel. Fixed-film systems do not lend them selves as easily to the application of mechanical mixers and again the additional cost for mixers and power are a factor in the economics of these anoxic/aerobic nitrification de-nitrification systems.
The typical anoxic/aerobic nitrification/de-nitrification system requires a 3:1 ratio of recycled nitrate rich water to influent wastewater to substantially reduce effluent nitrate concentration. At this ratio, a typical system can remove 70-75% of the available nitrates thus reducing the negative effects of nitrate-nitrogen on the receiving water systems. Increasing the recycle beyond the 3:1 ratio produces diminished returns because the effluent flow of the nitrification reactor will always contain nitrate-nitrogen at a concentration approximating that of the ammonia concentration entering the nitrification section. The nitrate removal effect is in fact created because the nitrate recycle stream, which does not contain ammonia at the end of the nitrification reactor, dilutes the influent ammonia concentration in and subsequently out of the de-nitrification reactor. The ammonia concentration of the liquid in the anoxic reactor does not vary greatly from the diluted concentration in the de-nitrification reactor but because it has already been diluted as it enters nitrification reactor, the nitrate-nitrogen concentration generated in the nitrification reactor is reduced.
The need to contact the nitrate rich final effluent wastewater with the bacteria in suspension or by forcing flow through the fixed media, or both, has always required the addition of mixing energy to the systems with mechanical devices, as is disclosed in U.S. Pat. No. 4,599,174, granted on Jul. 8, 1986, to Curtis S. McDowell, to maintain efficient contact between the wastewater and the biological fixed-film and/or to maintain the suspension of the facultative bacteria.
It would be desirable to provide a biological de-nitrification reactor that does not require additional mixers to maintain the required mixing of the wastewater and recycled nitrate stream and at the same time effectively contacts the mixture of these elements with the facultative bacteria. It would also be desirable to utilize the fluid flow and mixing energy provided by the nitrate recycle pumps to accomplish the required mixing of the influent wastewater and recycled nitrate stream and proper contact without requiring an additional input of mechanical energy or equipment.